The continuous increase in the isolation of bacterial pathogens exhibiting resistance to multiple antibiotics has created an urgent need for new therapeutic agents. Natural host-defense peptides and peptide fragments from antibacterial proteins of phagocytic host cells are currently being developed as antimicrobial agents. The pmrA locus of Salmonella typhimurium encodes a two-component regulatory system -PmrA/PmrB - that governs resistance to polymyxin B and antibacterial peptides/proteins of human neutrophils. Polymyxin B-resistant mutants have chemical modifications in the lipopolysaccharide (LPS) that make them less anionic, resulting in decreased binding of polymyxin B and cationic peptides/proteins of PMNs. Experiments are proposed to characterize in molecular detail the PmrA-activated genes which are required for polymyxin B-resistance and to examine the structure of the LPS in mutants defective in PmrA-regulated genes. The mechanism by which pH and divalent cation concentration modulate resistance will be examined by analyzing the interactions between the PhoP/PhoQ and the PmrA/PmrB two-component regulatory systems. The results from these studies should provide a detailed molecular picture for the capacity of Salmonella to resist cationic antibacterial peptides. Moreover, it may also lead to novel strategies to prevent expression of determinants that mediate resistance to antibacterial peptides, thereby allowing the host to clear bacterial infections.